Thursday's suspect packages were found in offices in Brighton, Slough and Oxford. Another suspect device was also sent to an office in Aldershot on Wednesday it was disclosed.

A military source described the first three sent as “basic, but viable” explosive devices. It was too early to say if the three found on Thursday were the same, but police sources said the devices were all linked.

Careers office staff and post office workers have all been put on alert after the incidents and told to be on the lookout for more devices.

Det Supt Stan Gilmour of the South East Counter Terrorism Unit said: “Because suspicious packages were found at various locations yesterday notes were sent out to all the Army Careers offices as well as the Post Office asking staff to be extra vigilant and look out for any suspect packages.”

The bombs discovered on Tuesday were found at offices in Reading and Chatham.

An MoD Spokesman said: “We can confirm that we are aware of security incidents ongoing in Oxford, Slough and Brighton involving Armed Forces Careers Offices. The relevant civilian police authorities are investigating, with military bomb disposal teams in support. Security advice has been reiterated to our personnel."